REVERSE OSMOSIS
By Matthew King

What is Reverse Osmosis?

Let's start out with some definitions: (please click on any of the key words listed below)

Ion
Osmosis
Osmotic Pressure
pH
Semi-Permeable Membrane
Total Dissolved Solids

Reverse osmosis?

How R/O Works

In this section, I will attempt to explain the mechanics of how reverse osmosis filters work.

First, the contaminated water passes through a filter (opening diameter of 1 micrometer), which removes large particles from the water (i.e.silt, sand, rust, dirt, etc).
Second, the water passes through an activated carbon filter to removes chlorine, pesticides, herbicides and toxic chemicals.
Finaly the water passes through the R/O membrane, which removes dissolved solids at the molecular level.

The purpose of the first two filters is to protect the R/O membrane form chemicals that could damage it or clog it. The water that makes it through the membrane is purified, while the contaminants and some wastewater are retained and recycled through the filter or disposed of.

R. O. Membranes

Disadvantages of Reverse Osmosis

the reverse osmosis process is not ion selective (removal of all ions, not just unwanted)
the reverse osmosis membranes concentrate the contaminants, which then require disposal
resultant waste percentages are about 20% - 25% of total flow
influent water may require prior treatment before entering the reverse osmosis system
the pH of incoming water is recommended to be 4.0, which often requires the addition of large quantities of sulfuric acid, sequestering agents and cleaning agents to the influent water
additives in themselves require special handling and containment considerations.

A serious problem for the reverse osmosis process is membrane fouling. Clogging may occur when high levels of harmless minerals, such as calcium and magnesium are present. Regular membrane replacement is needed.

Advantages of Reverse Osmosis

Reverse osmosis removes almost every contaminant and almost eliminates TDS. It is a well known remediation technique for groundwater; and it is becomming the easiest, cheapest and most energy efficient method for removal of dissolved solids from water (especially organics and nitrates).

Reverse osmosis most widespread applications are:

water purification for industries requiring ultra pure water (chemical ind., power ind.)
removal of nitrates from groundwater
and in home "under sink" water purification systems

Some R.O. Units:

Works Cited

Ground Water Pollution Table of Contents

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